In electrophotographic printing a common method of practice comprises the following process:
A latent electrostatic image is prepared on a photoconductive drum. Liquid toner (i.e., toner particles dispersed in a carrier liquid) is brought into contact with the photoconductive drum and toner particles adhere to charged (or discharged) portions thereof representing the latent image. Much of the carrier liquid is then generally removed from the image and background areas of the drum, by squeegeeing with a roller or other means as known in the art. The latent image is transferred by intimate contact to a drum (or other surface) that is comprised from material suited for high quality transfer of the image to a print media. The drum is referred to herein as an intermediate transfer member (ITM), since it serves to transfer the image from the photoconductive drum to the print media (e.g. paper or plastic).
An example of a system that operates utilizing this general methodology, is described in U.S. Pat. No. 5,410,392 the disclosure of which is incorporated herein by reference. High speed printers, such as the H-P 3000 sold by Hewlett Packard operate under this general methodology.
Optionally, in the process described above, a latent image can be repetitively formed on the photoconductive drum using different colored toners to form a multicolor image (for example, using cyan, yellow, magenta and black). In some processes the accumulation of the multicolor image is performed on the ITM and all of the color separations are transferred together to a final substrate. In some processes, each color is transferred separately to the final substrate, via the ITM.
Typically the ITM is heated in order to improve the quality of the image transferred to the print media. Heating of the ITM decreases the viscosity of the toner and ensures better transfer to the print media. In current practice, the toner solvates at least some of the remaining carrier liquid and becomes plasticized. The heat of the ITM typically causes the toner to coalesce and/or to form a film. This film is then transferred to the final substrate.
One method of heating the ITM, known in the art, is by installing a heater inside the ITM and heating the entire body of the ITM. This method requires generating enough heat to reach a desired temperature on the surface of the ITM in order to affect the toner. One drawback of this method is that heating continuously or repetitively to high temperatures has a detrimental effect on the resilience of the ITM and on the life of the coating of the ITM's surface. Furthermore, since the ITM is heated over its entire surface, heat loss and thus power requirements are high. Finally, heating an intermediate transfer roller from within is not efficient.
An alternative method that has been proposed for heating the toner on the ITM, is by radiating the surface of the ITM. However since many color inks have very low absorption rates this method requires the heating of the ITM surface coating, which will pass on the heat to the toner. In order for the surface coating to absorb the radiation and affect the toner, it is required that it be coated with a dark color (e.g. black). Additionally, radiating requires radiating at high temperatures due to heat loss. Further additionally, external heating has the risk of causing a fire in case of a paper jam in the area being radiated, due to the flammability of the carrier liquid.
As a practical matter, modern liquid toner printing uses an internally heated drum, covered by an intermediate transfer blanket.
One method of avoiding heating of an intermediate transfer member is to provide direct transfer of the image from the photoreceptor to the final substrate. To allow for film forming prior to transfer, the image is heated on the photoreceptor. However, since the photoreceptor itself is usually heat sensitive, the heating must be limited.
U.S. Pat. No. 5,426,491, the disclosure of which is incorporated by reference, describes a method by which the image on the photoreceptor is subject to heating, which causes the particle in the liquid toner to coalesce and form a film. However, heating using light is inefficient since in general only a minor portion of the energy is absorbed by the toner. Also, heating lamps are hot and pose a safety hazard in the presence of carrier liquid.
EP publication 0 549 867 A2, the disclosure of which is incorporated by reference, describes a method in which a heated electrified roller is applied to the wet image on the photoreceptor. The pressure and electrification of the roller cause liquid to be removed from the image and the image to be compressed and to become unitary. The heat causes the image to become tacky, so that it will adhere to the substrate. This publication makes clear that the method is meant as a way of avoiding the use of an ITM, which is considered to be undesirable by the inventors.